Communication system



July 4, 1944.

W. HASENBERG COMMUNICATION sisTEm Filed July 15. 1941 4 Sheets-Sheet 2 July 4, 1-944. w; HAS-ENBERG 2,352,711

COMMUNICATION SYSTEM Filed July 15. 1941 4 Sheets-Slieet s '7 E 7 O1 m 2 L1:

V \n 3 u 9 tutor: WEAA/EEf/HSENBERGI' July 4, 1944.

to oafpul va/ve w. HASENBERG COMMUNICATION SYSTEM Filed July 15. 1941 4 Sheets-Sheet 4 Patented July 4, 1-944 UNITED STATES PATENT GFFHCE COMMUNICATION srs'rmu Werner Hasenberg, Rio de Janeiro, Brazil Application July 15, 1941, Serial No. 402,510

7 Claims.

The invention relates to loudspeaking commu- 'nication systems and particularly to systems comprising a plurality of stations each equipped with transmitting and receiving means, and wherein the sound waves impressed on the communication channels provide the controlling means for eliminating the acoustic phenomena, known as singing or howling or alike, due to reed-back.

Loudspeaking communication systems are known to the art which avoid feed back phenomena by cutting down the volume of sound reception in both directions, but in my invention does away with such limitation and permits amplification to any desired degree. Another known loudspeaking communication avoids feedback by means of control'switches, apart from or combined with the line-selector switch, to be handor hand=relay-operated by the user or by the operator of a master station, who may thus interrupt or commutate the connections of a channel. In one variant of that system the electronic device, consisting of a mere amplifier, may also be made part'oi' the control system by connecting or disconnecting its power system supply and thereby making the amplification respectively operative or non-operative. In contradistinction to this system, my system is of the voice controlled type and does not require any switching except closing a line-selector switch; my electronic device comprises amplifying as well as control means; the latter being operated exclusively by the sound induced :urrents.

One object of my invention is to provide a system wherein any station or any number of stations may establish and maintain a two-way loudspeaking communication with any other station or any number at other stations without interference of feed-back effects. This result is attained without any regard to the character istics of the communication lines and in so far my invention is to be distinguished from systems which are based on a balanced repeater circuit.

more than two, say n stations, and is to provide a two-way communication between any two of them, each of the 72 stations must be equipped ting element, and, when a station has established actual communication with another station, part of the outfit either of the two destined to lay idle. A further inherent drawback of that system is that the station selecting another one can alone complete the two-way communication path without any active cooperation from the part of the counter-station, so that the latter may be connected with and overheard by a station without even noticing it. In contradistinction thereto, the present invention is based on the conception of independent station units, each self-contained with the outfit of a one-way transmitting or a one-Way receiving system only plus a. receiving element (loudspeaker) or transmitting element (microphone) respectively, so that in a system of n such units n-l channels lead to the one element, but one channelonly to the other element,

and any such station unit may start a one-way One important feature of my invention is to assign to each channel its own voice operated amplifying and controlling means independent of transmission to any other selected, while the opposite station has to complete the circuit into a two-way communication path, thereby enabling mutual control currents to reach their opposite amplifiers over an interconnecting auxiliary channel. It is another feature of my invention to reduce to a minimum the length and the cost of said auxiliary channel by-utilizing the sound transmitting channels for simultaneously conveying the control potentials. Thus, the intercormectingchannel can be reduced to a small bridge provided within each station unit, and any desired extension can be incorporated in a communication system by simply connecting the channels of the'new station unit or units without additional interconnecting lines or coupling devices.

The flexibility and adaptability thus afforded by my system permits of its connecting with the public local or long distance telephone or radio telephone system without afiecting their operative conditions or performance, by simply removing. the microtelephone oi the standard telephone set and introducing the free ends into one of my station units so as to transform the latter into a. terminal of the public system.

While amplifying and controlling meansfor suppressing feed-back'efiects are conventional to the prior art, the specific arrangement and coordination of these means which will be dethe restrictions heretofore imposed on the spatial arrangement of the microphone and loudspeaker of the station relative to each other by the necessity of avoiding acoustic coupling or feed-back. As practice has proved the means employed in my invention for suppressing any kind of teedback are so effective that microphone and loudspeaker may be arranged or assembled in the closest proximity without resorting to acoustic expedients, suchas, for instance, phase shifting of the acoustic waves which has been suggested by others, and the elements may even be combined into one loudspeaking transducer unit. The specific means for incorporating this unit in a communication system are another object of my invention.

When my invention is carried into eilect it may happen that the called-up station to which it is up to complete the two-way communication circuit by means of its selecting switch, may desire to convey a signal to the calling station before the two-way path is, or can be, established as, for

instance, in the case 0! being unaware of the identity of the calling station and, consequently,

unable to respond to the call. It is a special object of my invention to provide adequate means for using the one existing transmission path for the back transmission 0! signals or the like. Another object of my invention is to enable either party of the two-way communication established to cut-in or interrupt the talking of the other station.

These and other objects and advantages of my invention will be better understood from the following detailed description when read in connection with the accompanying diagrammatic drawings of which Fig. 1 shows a two-way communication between two stations wherein an auxiliary channel is used as interconnecting path for the mutual control currents exchanged between the station. Fig. 2 shows a complete communication system comprising 4 stations wherein any two may simultaneously establish and maintain a twoway communication in the manner of Fig. 1 and the channels are serving as transmission paths for sound induced currents as well as for control currents and a small bridging lead provided within each stationunit takes the place of the auxiliary channel.

Fig. 3 shows the kind of electronic device preferably employed in my invention and associated with the station units in the manner shown in Figures 1 and 2.

Fig. 4 illustrates the means for enabling a station called-up to convey signals back to the calling station over the one existing'path; and further means for cutting-in or interrupting the talkoi the. other station after the completion of the two-way-communication path.

5 represents aj'transducer and means; for it in a communication system,and I Fig. Gillustrates afurther improvement otthe electronicdeviceshowninl'im3byaddingan In dealing with each figure in detail 1 shall first explain the significance of its numerals and then proceed to describing the operation of the system or arrangement represented by the figure.

As far as practicable the same numerals have been used to denote corresponding elements in the difierent figures, and the same letter indices to denote elements belonging to the same station. When, however, such elements are discussed under'their typical aspect, without reference to an individual station to which they belong, the letter indices will readily be omitted and it will be understood that the numeral under discussionis represented in the figure by any one of its like numerals bearing a letter index.

In Fig, 1 Il1-a and ill-b represent two stations comprising microphone li-a, H-b, loudspeakers i2-a and "-1), switching devices l3-a and l3-b and terminals Ill-a, l5--a, l6a, ll--a and l4-b, l5--b, lG-b, l'l-b. Between the two stations are running channels I8, i9, 20 and 2| for transmitting from and receiving at a station, with electronic devices 22-41 and 22-h inserted in these channels. The electronic devices which will be described in detail later on in connection with Fig. 3, comprise ampliflers and voice operated control means necessary for eliminating all kinds of feed-back effects and maintaining a two-way communication in an orderly way. The output 01 the electronic de 1 vice 22-0 is in fixed connection with the receiving channel 2| of station Iii-a and correspondingly the output of electronic device 22-!) with the receiving channel is of station iU-b,

while the microphone lla of station Ill-a may be connected by means of switch i3-a through channel is to the input of electronic device 22-1), and correspondingly the microphone "-17 of station Ill-b by means oi switch l3-b through channel 26 to the input of electronic device 22-41. The general character of the arrangement is so far conventional were it not for the interconnecting channels indicated by the dotted lines 23, 24, 25and 26.

The operation is as follows: When station i0a wishes to start a two-way communication with station ili-,-b, switch i3--a is to be closed. The sound waves impressed on microphone H--a are. transformed into voice induced currents which travel along channel is to the input of the electronic device 22-h, where they are amplified and Iarther from its output travelling along channel is reach loudspeaker l2b, thus constituting a one-way transmission from station ill-a to station ill-b. It is now up to station.

iflb to turn the one-way transmission so far established into a two-way communication by completing the circuit which is done by closing switch l3-b. As soon as the two-way path is.

induced current which is then carried'on thru channel is to the receiving station ill-b, simultaneously produces a control potential which is conveyed thru auxiliary. channel 23 to terminal (l-b and over the Il -b, now-closed, to

terminal 4.1mm there that auxiliary'chlnne! :4 tottm-mputotelectmnicmice lI-c whereit operates to block any from m-m, station "-6. whenstntionil-b the, two-way communication.

1.85mi transmitting'sound to Ill-u. a c oi s Q trol potential is produced byelectronic device fl-a and conveyed to device 22-h thrmthe auxiliary channels 2 and 2 by ,theintermediary of the nowclosed switch |I-a As soon as the transmitting station stops talking, the initial condition, is automatically lie-established, and the path free again for -a transmission to be from the other station.

Summarizing, the control produced at one electronic device and intended to act uponv the other electronic-device so as to block its'transmitting path of which .it is forming part itself. is conducted from one to, the other by mutual auxiliary channels which are conpotential which is Werner-f 1H or station n-e is infixed L connectionwiththe-outputterminals ll-a of the electronic device Il-aover channel H and the corresponding elements of the other sta- 'tions, alike. T e input terminals -001 the electric; device 2l-.-a are connected to channel "-aleading-to the switching device of station I'L-b and so on. It' wlll'be noticed that -.thei.

, auxiliary channels" of Fig.'1'are lacking here. being partly su zo'erseded by the bridges 343 which "interconnect the-center tap of the line -side transformer windings Si and 32 and thereby the nectedo'nly when the counterstation completes This. is an important feature of my invention inasmuch as it permits building up of a system comprising a plurality of stations .in the simple way which will be more fully, explained in connection with Fig. 2, and to establish and maintain a loudspeaking two-way communication between any two of these stations by means of switch It to be equipped with the corresponding number of L contacts for selecting the respective channels.

Each station proper, circumscribed in the figure by a rectangle, together with its electronic device and that of the two channels which provides the fixed connection without inserted switch'between the station and its electronic device, constitute what may be defined as a "station uni It will be understood that either station unit is self-contained in so far as it.

aifords: its own. amplifying and control means and the combihation of two such independent transmitting channelliF-b with the receiving channel 3H. which take over the other part played bythe auxiliary channel in Fig.1.

The'twoway communication shown in Fig. 2'

operates as follows:-

when station 21-4 happens to be transmittingto station li -b, the first is making use of channel lb leading to the input terminals "-12 of the electronic device 28-h, where the a sound induced currents are amplified and also station units into a two-way communication I system requires nothing but a few simple line connections without addition of a linking device apart from that of the interconnecting auxiliary channels. The latter, however. which obviously involve an expenditure increasing in pro portion to the number of station units combining. may'still be eliminated .to .a large'extent by using the-proper communication channels as transmission paths for the control currents as well as for the sound induced currents. This part of my invention is explainedin Fig. 2 together with the application of my system to a plurality of 4 stations or station units between produce a control potential; This potential is delivered by lead-Il-b to the center tap of the transformer and its'output terminals "-1) and from there, always together with the amplified sound currents, conveyed into channel "-1). which is the receiving channel of station 21-! leading to transformer 32-!) of that station. The sound induced currents are received here and re-transformed into sound by loud-speaker "-12. whereas the controlpotentiai is conveyed farther on over the bridge 04--b and transformer winding Ii-b into the opposite channel 4H of the two-way path thru which it finally reaches the input-terminals 38-4 of the electronic device Il-c and over'wire "-0 can act upon the elements of its inner circuit.

when vice-versa, station 21-!) is transmitting to station 21-11.. the process is analogous, the

path along the control potential is conveyed be-' ing 80-4, 21-4. I2-a, it-a, Il-a. lt-b,

' lS-b, 2H. "-17.

any two of which the two way communication" may at any time'be ambu hed.

Thesystem -of Fig.2 comprises 4 stations 21-11 to 21-11 and 4 electronic devices 18-0 to 28-11. Each stationconsists of microphone 2! and loud; speaker 30, either -oi which are connected to transformer II and 32 respectively. and switching device 33 provided with contacts for 3 different channels. The centertaps of the line-side windings of these transformers are interconnected by lead ll.

The electronic devices :s-tne system or which ,will be described/later on in connection with Itwill thusbe understood that the control po- E tential introduced into one station thru its receiving channel together with the sound currents is automatically conveyed farther on over the bridge to the other channel which completes the path of the two-way communication with a the other station. Suitable means must,- of course, be provided for preventing any coincidence of oscillations between the sound induced and the control currents along their common path, preferably by rectifying and filtering the sound induced currents in the way illustrated in Fig. 3 illustrates the arrangement of coordination of the amplifying and controlling means embodied in the electronic device. A device to this. description has givengood results and is.

preferably used when this invention is carried into efi'ect although I do not limit myself to any of the details given herewith. I The device comprises a primary circuit or mai path, and a secondary circuit or by-path. The

primary circuit comprises valves ll and 42 which constitutes two-stage sound amplifier, the secondary circuit valves 43 and M. the latter valve being of twin-diode-pentode type. For simplicity the conventional heater circuits of the thermionic valves have been omitted. I Both, the primary'and the secondary, circuits have a common input, the control grids of the input valves being shunted for alternating current by a capacitor 4| and a common grid leak resistor 48 for bias voltage.- The input transformer 41 has its primary 41' connected to the input terminals 35 and its center tap to lead 31, which is connected to and 43 are joined and thus charged with the same positive potential over a voltage divisor consisting of the resistances 50, 5i and 52.

The secondary circuit further comprises conventional means for controlling the gain, such as a potentiometer 53 in the control grid circuit of the second valve 44 and a variable resistor 54 inthe cathode circuit of that valve, the resistor being designed for determining the threshold conductivity of the diodes 55 and 56.

I shall now describe what is taking place either when sound induced currents or when a control potential are introduced in the electronic device.

When sound induced currents reach the input terminal 35 of the electronic device and follow the path of the secondary circuit, they are first amplified by the valves 43 and 44 and then after passing thru capacitor 51, are rectified by the shunted diodes 55 and 56 so that a rectified D. 0. potential is supplied for charging condenser I over resistance 5'! with a negative value. This potential is farther conveyed over the RC- filter 64, BI and wire 38 to the center tap of the windings 12" of the output transformer 12 and thus to the output terminals 38 of the electronic device. The output circuit of diodes ll further comprises a voltage divisor consisting of II. The junction of these two resistances supplies the control potential proper which over the filter '2, 83 and wire it is conveyed to the control grid 61 of valve 6!. Wire 66 and valve 8| thus connect the by-path back to the main path. It is the function of valve 68 to block and unblock the main-path in the following way. Anode of valve 68 is connected directly to the second'grid of valve 4| and fed over resistance 'the resistances 59 and t0 shunted by condenser II and conventional filter elements from a con ventionai power supply of which only the terminals are shown in the figure, its positive pole belng connected to terminal ll, its negative, pole grounded. The cathode resistance 13 provides the normal bias potential adjusted to about 1 volt. for grid 61 of valve 68. Under these. conditions a voltage drop caused along resistor ll by the plate current from anode reduces the positive potential at'the second grid of valve 4| to a limit where this valve becomes practically unoperable, in other words, is blocked. The unblocking of valve 41' takes place as soon as valve II is blocked due to a control potential biasing grid 61. The rate at which the main-path or primary circuit is unblocked in response to the sound induced currents impressed on the input of the electronic device, apart from'the adjustment of the RC-time-constant elements substantially dependupon both, the gain of the secondary circuit amplifier and the threshold value of the rectifier. By appropriately adjusting these limits, the controlling means can be made unresponsive to room noises and the like, so as to make the electronic device operative exclusively for the transmission of the proper sound currents.

When a control potential reaches the input terminals ll of the electronic device and follows the path of the secondary circuit, it conveyed over leads 31 and 48 to junction point 4!, grounded over resistance 48, and biasing the control grids of valves 4i and 4|. The gain of valve 4! is thereby reduced to complete blocking making the by-path inoperative. The reason for biasing the control grid of valve 4| in the main-path simultaneousiy with that of valve 43 in the bypath is that I prefer'this method for making the overhanging time long enough to keep the gain relatively constant during the intervals between the syllables, and insuring a-smooth overcoming when the first station ceases talking and I the second starts immediately after. Valve 4i thus becomes inoperative in less time than if the blocking should be determined only by the decrease of the potential charging grid 81 of valve I shall now describe the function which the electronic device has in the operation-cf the twoway communication established between stations 21-1: and 2lb according to Fig. 2 wherein electronic devices 28-a--b-cd must now be understood as embodying the full diagram of Fig. 3. Whenthe user of station 21 -11 starts a conversation, the currents induced by his voice reach the input 35-h common to the primary and secondary circuit of the electronic device "-1), and

are carried on, as shown in Fig. 3, over the transformer 41-1: to the control grids of both valve 4 l-b in the primary and 43-h in-the secondary circuit. Valve 4l-b is in a state of non-conductivity, amplification of the sound currents is,

therefore, possible only along the secondary circuit by valves 43-b and 44-b. The diodes 55b/56b of valve 44--b become conductive and charge the upper plate of condenser "-11 negatively. Over the back connection 88 to the primary circuit this charge is applied togrid 61-11 of valve 68b, the bias potential of which is increased, so that valve 4l-+b is unblocked and amplification of the sound currents can now take place in the primary circuit being opened for transmission and deliveringthe amplified currents thru the output "-12 of-the electronic 7 device into channel 39b. Simultaneously the charge of the upper plate of condenser .r-bl! conveyed to winding I2"--b'of the output trans-.

former II-b and into the receiving channel 39-!) wher it joins the amplified sound induced current whose path so far has just been described, and both together are conducted to station 2|--b over the recelving channel "-1) being their common transmission path. At statlon -2Ib the sound induced currents are retransi'ormed into sound by the loudspeaker "-b while the control current is carried farther onover lead 34-h andthe line'side winding of transformer 3Ib into the transmitting channel.

48-0 and reaches the input 35-4 of the other electronic device 28a. Within this device, the D. C. control potential is conveyed from winding 4'l'a of the input'transformer 41-0 over leads 31-1; and 48-4 along resistor 49-a to the ground common to all electronic devices of the system. The voltage drop produced alon resistor 49a has the effect of biasing the control grids of both, valve-4H, thereby blocking the secondary circuit, and valve 4l-a, thereby insuring against unblocking or the primary circuit. The path over which sound energy should be transmitted from station 21-!) to station 21-0 in reply to the latters ca l, thus remains inactive being blocked as long as' station 21-11 is talking. This has the advantage that the objectional efiects of feed-back, et cetera, are entirely eliminated, but is also a handicap insofar as it does not permit the user at station 21-!) to interrupt or cut-in while the other is talking since his own transmission path is still blocked. In these circumstances interrupting would only be possible when the intervals 'or pauses between the syllables or words or sentences happened to be relatively large. For enabling the receiving station to cut-in or interrupt at will, I have provided additional arrangements which will be described later on in connection with Fig. 4.

The function of the electronic device in the operation of the system when used in the opposite direction, is analogous to that in the direction described. I

The time which takes the unblocking of the initially blocked primary circuit from the very moment of sound induced current reaching the input of the electronic device, can be made so small as to be practically unnoticeable, i. e., 0.08 to 0.12 second. The control potential which makes the, secondary circuit of the other electronic device inoperative may be adjusted at 7 an average value of two or three times that refactor, I may add to the by-path an automatic amplitude responsive gain reducer, the arrangement of which is shown in Fig. 6. 4

Fig. 6 represents a detail out of the total system shown in Fig. 3, whereof those elements and those elements and connections reproduced only ing in that in the normal position switch 13 is closing and 14 opening its circuit. Switch I3 is inserted in the receiving channel I 39, switch 14 is arranged for connecting a. line 80 to the transmitting channel leading to that electronic connections have been omitted which are not reof valve 256 and also grounded for A. C. over the capacitor, 95. Cathode 9i of diode 90 is connected over a capacitor 94 to the control grid of th input valve 2 of the primary circuit and over resistor or choke 93 to the movable arm 91 of potentiometer 25!.

The circuit operates as follows. Cathode Si is charged with a positive potential which can be so adjusted as to make the diode conductive when the voice talking into the microphone con-.

nected to the input terminals 235 becomes too loud, but for all amplitudes not exceeding a predeterminable level of sound intensity, the diode 9n remains non-conductive. .When the sound energy exceeds such limit and diode '90 becomes conductive, a continuous current can flow along resistor or choke 95 and charge the control grid of valve 246 more negatively. This operates to reduce the gain of this valve'up to complete blocking and-has the consequence that no D. C. control current can be delivered Supplementary to the control exerted on the control grid of valve 243 by the D. C. control voltage theautomatic amplitude responsive gain reduction controls'the by-path so as to keep amplified sound induced currents from-reaching the diodes 255/256 whenever the input valve 243 tends to chan e from a type A-amplifier to a B or C type.

Fig. 4 represents modified details of Fig. 2 with 75 from diodes Additional to the circuit shown in device whose output is in fixed connection with the station embodying the switches that is in the example shown by Fig. 4, to transmitting channel l40a leading to electronic device 8-11 the output of which is in fixed connection with station l21-a thru the receiving channel I39-a. Line at represents a distribution system branched towards all stations of the communication system and is permanently energizedby both a sound-signal generator 15 and a constant D. C. current source 8i, suitably combined into one unit I5 equipped with an output transformer Tl, whereby the D. C. source is grounded at one end and connected to the center tap of the line side coil of transformer Tl at its other end. The function of switch 13 is to enable the called up station to cut-in or interrupt while the calling station is talking. Let l2lb be the calling station just transmitting to the called-up station l21a. When the latter desires to cut-in, all he has to do is to press the button of switch l3--a start talking and then again release the press button. This short interruption of the receiving channel cuts 015: the sound current, but at the same time, the control current due to the talking of station l2'l-b, since both currents have been entering station Ill-a on the common path I39 a., so that electronic device l28--b is no more reached by the blocking potential, the forterruption is a shifting-over of the command of the transmission path from the talking to the interrupting station, and the system is now normally operating in the reverse sense, i. e., under same conditions as if i2|a had been the calling station and l2|--b been called-up.

In other conditions, the interruption of a channel while a station is talking, may well affect the stability of a system by giving rise to feed-back effects and dangerous peaks in the opened transmissionpath, but in my invention both are suppressed by the immediately and automatically effected reversal of the operation of the control system, provided the interrupting switch is associated with the receiving channel.

In order to explain the function of switch 14 I shall exemplify one of its possible applications with reference to Figure 4.

Assuming that station l21-b has called up station l2l--a. Microphone l29-b which may preferably be of the moving coil or another reversible .type, is then switched on to channel ll-a leading to the" input transformer of the electronic device l28-a. The call has been received by station l2'|-a, but for some reason or the other, station |2'|-a has not become aware of the identity of the calling station, therefore in order to respond to the call, would have to try out by calling up one station after the other. Instead of this complicated and time-taking procedure station |2la is enabled to transmit back a preconcerted signal to station l2'lb over the one existing one-way path by pushing the button of switch "-a and thereby connectin channel 8-41 to line 80. The microphone l29-b thus becomes the receiving element for the sound signal conducted thru this channel and the electronic device i28a becomes non-operative since the D. C. voltage simultaneously conveyed thru-channel Mil-a has a blocking effect on the secondary circuit of device l28-a. The signal can only be received by that station which has established the one-way path but not by any other station and ceases when the switch button is released. The user of station l2'lb when receiving this preconcerted signal will understand that he is asked thereby to reveal his identity.

It will be understood that the value of the D. C. voltage supplied by source 8| must exceed the sum of the cut-off bias potential of valve 43 in the secondary circuit plus the peak value of the signal, otherwise valve 43 may change from an 'A type into a B type or C type amplifier and, as

such fail to effect total blocking. Numerically this value may be about 25 v. being negative with respect to the line.

Figure 5 shows the transducer 8| which is a combination of the receiving and transmitting element into one unit and the means for incorporating this unit in the communication system. The moving coil of transducer 8! constitutes one leg of a bridge-network. The second and. third leg are formed each by one-half 82' i and 82" respectively of the station-side winding the automatic amplitude responsive gain reof transformer 82, the line-side winding being 1 connected to the terminals 81 of'the receiving channel. The fourth leg is formed by the induction coil 86. The balanced points 84 and 85 of the bridge are connected to the station-side windings 83' of the transformer 83 whose lineside winding 83" is conne'cted to the terminals 88 of the transmitting channel. Lead 89 represents the bridge which is identical with the one denominated 34 in Figure 2.

The operation is as follows: Accoustic waves impressed on transducer 8! and transformed into sound currents are conducted over transformer 83 and terminals 88 into the transmitting channel. The problem of incorporating the transducer unit in the communication system is to avoid feed-backs from= thev receiving channel owing to both, the receiving and transmitting element, being electrically connected to each other within the transducer. This'is achieved by the bridge-network. It is evident that if the bridgenetwork is balanced, no sound currents from the receiving channel 8'I-can reach the transmitting channel. The balancing may be carried out by ducer circuit illustrated in Fig. 6.

Modifications of the systems of my invention described and illustrated above, may be apparent to persons skilled in the art. I wish to make it clear that all details are given by way of examples. only and do not limit in any way the spirit of the invention and the scope of the appendent claims.

I claim:

1. In an arrangement for establishing and maintaining a simultaneous two-way communication of the loud-speaking type in a network of independent one-way transmitting channels, thereby preventingany kind of feed back, a loudspeaking reproducer and a voice transmitting element, belonging to different and remotely located stations operatively associated with each channeheach channel having its own amplifying and electronic controlling means for providing transmission of sound above a predetermined threshold level, thereby producing a control current which enters the channel simultaneously with the amplified sound current, and disabling means responsive to control current fonproviding and maintaining the initial non-conductivity of the channel for transmission even for sound intensity above said predetermined threshold 1e-' vel, which comprises separate receiving and transmitting circuits at the stations adapted to be connected to transmitting channels of opposite direction for simultaneous two-way communication, interconnecting means between said two circuits at the stations constituting a bridge for control current to pass over from one into the other circuit, so that control current impressed together with amplified sound current at the receiving circuit at the station enters the channel of opposite direction which has been connected to said transmitting circuit of that station.

2. In a voice controlled, loudspeaking intere,-

communicating system; the combination including, an equal number of stations, one-way trans mitting channels and electronic devices, each electronicdevice having means responsive to sound induced current impressed on its input, in-

eluding thermionic valves, for amplifying and rectifying, providing amplification of sound current and producing a control current respectively, gain controlling means responsive to sound in-' duced current signals above a predetermined energy level operatively associated with said first mentioned means, said gain controlling means adjusting coil 86 or in any other conventional way.v The stability of the two-way communication system is thus maintained even with slight deviations from perfect balancing provided that the highest resultant A. C. amplitude sent into the transmitting channel does not exceed the value of the D. C. control voltage conveyed thru the same channel. False operation due to excess values may be avoided by making use of being also responsive to control current impressed on said input of the device for disabling said first mentioned means, an output circuit adapted to be fed simultaneously with control and amplified sound currents produced by said amplifying and rectifying'means in response to sound induced current input signals above a predetermined energy level, said electronic device itself not being conductive for transmission of sound of input signals below a predetermined energy level not of those above said predetermined energy level 4 I since there is simultaneously a control current impressed on said input of the device, each of said transmitting channels being associated with its own electronic device, each station including separate receiving and transmitting circuits leading to a loudspeaking reproducer and a voice transmitting element respectively, means interconnecting said receiving and transmitting circuits constituting a bridge for control current to pass over from one into the other circuit, the receiving'circuit being permanently connected to the end of that of the channels to which it belongs, and switch means at the station for selectively connecting said transmitting circuit to selected transmitting channels for transmitting to selected stations.

3. In a voice controlled, loudspeaking communication system, the combination including, a

. number of stations, each having separate receiving and transmitting circuits leading to a loudspeaking reproducer and a voice transmitting element respectively, interconnecting means between said two circuits constituting a bridge for control current to pass over from one into the other circuit, and switch means for selectively connecting said transmitting circuit to selected communication channels; communication channels associated with electronic devices, each electronic device having amplifying and rectifying means, including thermionic valves, for delivering simultaneously amplified sound current and a control current at the output of the device in response to sound induced current signals above a; predetermined energy level impressed on the input of the device, and for maintaining the initial non-conductivity even. for transmission of sound input signals above said predetermined energy level during the period of presence of control current at said input, said stations and communication channels operatively associated with each other so that a two-way loudspeaking communication may be established between any two stations over two of said channels, the two-way communication itself being maintained and controlled by mutual control currents flowing from the output of the electronic device of that of the two channels which is actually transmitting sound, over the bridge at the station into the sec and channel and on to the input of the second electronic device, the channels themselves serving as a transmission path to sound induced currentsas well as control currents.

4. In a voice controlled, loudspeakingcommunication system, the combination including a number of stations each having separate receiving and transmitting circuits leading to a loudspeaking reproducer and a voice transmitting element respectively, interconnectingv means between said two circuits constituting a bridge for control current to pass over from one into the other circuit, switch means for selectively conmeeting said transmitting circuit to selected communication channels, communication channels associated with electronic devices, each electronic device having amplifying and rectifying means, including thermionic valves, for delivering simultaneously amplified sound current and a control current at the output of the device in response to sound induced current signals above a predetermined energy level impressed on the input of the device, and maintaining the initial non-conductivity for transmission of input signals above said predetermined energy level during the period of presence of control current at said input, said stations andcommunication channels operatively associated with each other so that a two-way loudspeaking communication may be established between any two stations over two of said channels, the two-way communication itself being maintained and controlled by mutual control cur rents flowing from the output of the electronic device of that of the two channels which is actually transmitting sound, ,over the bridge at the station into the second channel and on to the nels themselves carrying control as well as sound induced currents, a break-in switch at the station inserted in said receiving circuit ahead of said interconnecting means for momentarily interrupting' reception of both control and amplified sound currents and re-establishing conductivity of the other channel while transmitting to the other station and consequently providing nonconductivity of the first channel which has been transmitting. I 4

5. An electronic device, including in combination, two parallel circuits operatively associated with each other, the primary circuit constituting a sound amplifier for amplifying the sound to be transmitted, said amplifier having at least the input valve on the pentode type, one of its grids being connected both directly through a lead to the plate of an associated three-electrode valve and over a high resistance to the power supply,

while the control grid of said associated valve is connected over filter means to a voltage divisor in the output circuit of a rectifier belonging to the secondary circuit of the electronic device, the secondary circuit itself constituting a detector amplifier, includin a diode, and means for adjusting the gain'of the amplifier part therein as circuits of both the detector and the sound amplifier for delivering simultaneously amplified sound current and a control current at the output terminals of the device in response to sound induced current signals above a predetermined energy level impressed on the input terminals, means associated with the input circuits of both the sound amplifier and the detector amplifier responsive to control current impressed on the input terminals of the device for disabling production of control current by said detector amplifier and consequently providing non-conductivity of the device itself for transmission of 'sound, even for sound current input signals above said established predetermined energy level.

6. A station for use in an intercommunicating system, including in combination, separate receiving and transmitting circuits responsive to voice currents as well as to interstation control currents means interconnecting said two circuits constituting a bridge for control current to pass over from one into the other circuit, a loudspeaking reprodu'cer operatively associated with said receiving circuit, and a voice transmitting element operatively associated with said transmitting circuit, switch means for selectively connecting said transmitting circuit to selected communication channels for transmitting thereof, and a break-in switch inserted in said receiving circuit ahead of said interconnecting means for momentarily interrupting reception of incoming control and amplified sound currents, thereby preventing input of the second electronic device. the chem that control current enters said transmitting circuit.

'7. A station for use in an intercommunicating system, including in combination, separate receiving and transmitting circuits responsive to voice currents as well as. to inter-station control currents, said two circuits constituted substantially by the line side windings of transformers, means interconnecting the centertaps of said transformer windings for conveyin control current I of the moving coil of the transducer, a balanced bridge network, the four legs of which being constituted by the moving coil, said inductance coil and the symmetrical halves of the station side winding 0! the receiving circuit transformer, the

station side winding of the transmitting circuit transformer being connected to the centertaps of the station side winding of the receiving circuit transformer and to the junction of said two coils respectively, switch means for selectively con- 30 necting said transmitting circuit to selected communication channels for transmitting thereover.

a break-in switch inserted in said receiving circuit ahead of said interconnecting means for momentarily interrupting of incoming control and amplified sound currents, thereby preventing that control current enters said transmitting circuit.

WERNER HASENBERG. 

